Unfortunately, in most cases, the data distribution is unknown.
Traditional algorithms make use of some prior information to infer this distribution.
For example, LZ77 [1]} and LZ78 [2]} used by most archive formats assume that data with lots of repeated segments occur more often than those without. For image compression algorithms, PNG [3]} assumes most adjacent pixels are similar to each other, and JPEG-like algorithms apply the fact that lower frequency components are more significant than higher frequency ones.
